Many machines, such as vehicles, sometimes operate an internal combustion engine to provide engine-braking, where the engine consumes external energy (e.g., vehicle momentum). One type of engine-braking—compression-braking—uses the external energy to drive a piston of the engine during its compression stroke and then releases compressed gas from the combustion cylinder to reduce the amount of energy returned to the piston during the subsequent expansion stroke. To release compressed gas from the combustion cylinder, the engine typically opens a valve with one or more valve actuators. In many cases, the engine releases compressed gas from the combustion cylinder around the end of the compression stroke and/or the beginning of the expansion stroke. For example, some compression-ignition engines with unit-type fuel injection systems employ an injector cam of the injection system and a hydraulic actuator to open a valve around this part of the cycle.
Over the course of the compression stroke of the piston, the pressure in the combustion cylinder progressively rises, often reaching very high levels at the end of the compression stroke. The valve actuator(s) that open a valve to provide compression-braking must work against this pressure in the combustion cylinder, which generates stresses in the valve and the valve actuator(s). Depending on various parameters, the forces required to open the valve against the elevated pressures at the end of the compression stroke and/or the beginning of the expansion stroke may create undesirably or unacceptably high stresses in the valve actuator(s). Additionally, the higher the pressure rises in the combustion cylinder at the end of the compression stroke, the more noise it may produce when released, which may cause undesirably or unacceptably high noise levels from the engine during compression-braking mode. These concerns may prove particularly difficult to address in cases where concerns unrelated to compression-braking limit the ability to adjust the timing of the valve opening for compression-braking. For example, in applications that employ an injector cam to open the valve for compression-braking, considerations related to injector timing may limit the ability to adjust the valve timing to reduce stress in the valve actuator(s).
Published U.S. Patent Application No. 2008/0223325 A1 to Meistrick (“the '325 application”) discusses a method that purportedly provides engine-braking with reduced forces on the valve actuator(s) used to implement the compression-braking, as well as reduced noise. Specifically, the '325 application discusses an engine-braking approach referred to as “bleeder type engine-braking.” According to the '325 application this approach involves opening one or more valve(s) early in the compression stroke and holding them open at a constant level for an extended period. The '325 application states that this approach provides reduced forces on the valve actuator(s) used to open the valve(s) and noise output of the engine.
Although the '325 application discusses an engine-braking approach that purportedly reduces force on the valve actuator(s) used to implement it and noise output of the engine, certain disadvantages may persist. For example, holding the valve(s) open for an extended period may tend to reduce the amount of work the piston performs during the compression stroke, which may tend to compromise the amount of engine-braking power provided by the engine. Additionally, “bleeder type engine-braking” may not lend itself to certain applications, such as, for example, applications relying on an injector cam to open one or more valve(s) for compression-braking.
The compression-braking system and design of the present disclosure solves one or more of the problems set forth above.